It is required for some driving units of e.g. copying machines that the output shaft be always rotated in the same one direction, irrespective of whether the input shaft is rotated in one or the other direction by a rotary driving source or a reciprocating driving source. Some driving units of this type use a rotation transmission device to achieve this purpose, i.e. to convert rotation in either direction applied to the input shaft from a driving source to the rotation of the output shaft in a single direction.
FIGS. 13 and 14 show a conventional rotation transmission device of this type (which is disclosed in JP patent publication 2004-84783A). As shown in FIG. 13, this rotation transmission device comprises input and output shafts 51 and 52 which are coaxial with each other, a pair of opposed bevel gears 53 and 54 rotatably mounted around the output shaft 52, an intermediate bevel gear 55 meshing with both bevel gears 53 and 54, a first one-way clutch 56 disposed between the input shaft 51 and the output shaft 52, a second one-way clutch 57 disposed between the input shaft 51 and one of the opposed bevel gears 53, and a third one-way clutch 58 disposed between the other of the opposed bevel gears 54 and the output shaft 52.
As shown in FIGS. 14(a) to 14(c), the one-way clutches 56, 57 and 58 each comprise an outer ring 59 having a plurality of inclined cam surfaces 60 on its inner periphery, rollers 61 disposed at the circumferential positions where there are the respective cam surfaces 60, and springs 62 biasing the respective rollers 61 toward the positions where the rollers 61 are locked by the respective cam surfaces 60. Of these clutches, the first and third one-way clutches 56 and 58 are mounted such that when the outer ring 59 rotates in the normal direction (counterclockwise direction in the figures), the rollers 61 are locked by the respective cam surfaces 60, while the second one-way clutch 57 is mounted such that when the outer ring 59 rotates in the reverse direction (clockwise direction in the figures), the rollers 61 are locked by the respective cam surfaces 60.
Thus, when the input shaft 51 or the bevel gear 54, which are rotationally fixed to the outer rings 59 of the first and third one-way clutches 56 and 58, respectively, rotate in the normal direction, or when the output shaft 52 rotates in the reverse direction, their respective rotations are transmitted through the first and the third one-way clutches 56 and 58. When the input shaft 51, which is rotationally fixed to the outer ring 59 of the second one-way clutch 57, rotates in the reverse direction, or when the bevel gear 53 rotates in the normal direction, their respective rotations are transmitted through the second one-way clutch 57.
Now description is made of the operation of this device when the input shaft 51 is rotated by a driving source. In FIGS. 13 and 14, the solid arrows indicate the rotational directions of the respective elements when the input shaft 51 is rotated in the normal direction, while the arrows of one-dot chain lines indicate the rotational directions of the respective elements when the input shaft 51 is rotated in the reverse direction.
When the input shaft 51 is rotated in the normal direction, its rotation is transmitted to the output shaft 52 through the first one-way clutch 56. At this time, since the rotation of the input shaft 51 in the normal direction is not transmitted to the bevel gear 53 through the second one-way clutch 57, and the rotation of the output shaft 52 in the normal direction is not transmitted to the bevel gear 54 through the third one-way clutch 58, the bevel gears 53, 54 and 55 all remain stationary. When the input shaft 51 is rotated in the reverse direction, the reverse rotation of the input shaft 51 is transmitted to the bevel gear 53 through the second one-way clutch 57, and then transmitted to the bevel gear 54 through the intermediate bevel gear 55, thus rotating the bevel gear 54 in the normal direction. The rotation of the bevel gear 54 in the normal direction is transmitted to the output shaft 52 through the third one-way clutch 58. At this time, since the input shaft 51 is rotated in the reverse direction, and the output shaft 52 is rotated in the normal direction, no rotation is transmitted through the first one-way clutch 56. Thus, this rotation transmission device can rotate the output shaft 52 always in the normal direction irrespective of whether the input shaft 51 is rotated in the normal or reverse direction.
But since the rotation transmission device disclosed in JP patent publication 2004-84783A includes three one-way clutches, a large number of parts are necessary. Also, during operation, the clutches tend to become out of alignment with each other, which causes uneven transmission of torque and increases operational torque.
Many of today's copiers, printers and other business machines have the function of printing both sides of each sheet of paper to conserve paper. In order to print both sides, a mechanism is necessary for reversing the feed direction of paper sheets. One of such mechanisms includes two driving sources, one for rotating the paper feed roller in the normal direction, thereby feeding the sheet in the paper discharge direction, and the other for rotating the paper feed roller in the reverse direction, thereby feeding the sheet in the reverse direction (JP patent publication 2002-154727A). Another mechanism uses a spring clutch to simplify the mechanism and reduce the cost (JP patent publication 2009-8247A).
The rotation transmission device using such a spring clutch includes first and second input gears that are coaxial with each other. When driving torque is applied to the first input gear, the clutch spring, which is provided on the first input gear, becomes locked, thereby rotating the output gear in the same direction as the first input gear, and thus rotating the paper feed roller in the normal direction. In this state, the second input gear is kept disengaged by e.g. an electromagnetic clutch, so that no driving torque is applied to the second input gear. Thus, the first input gear and the output gear rotate in unison. In this state, when driving torque in the reverse direction is applied to the second input gear by activating e.g. the electromagnetic clutch, the spring clutch of the first input gear becomes disengaged by the second input gear, so that the output gear now rotates in the same direction as the second input gear, thereby rotating the paper feed roller in the reverse direction.
With this rotation transmission device, it is necessary to provide the first input gear with the spring clutch and also to provide means, such as an electromagnetic clutch, for cutting off transmission of driving torque to the second input gear while the first input gear is being driven, these members add to the number of parts of the rotation transmission. It is thus difficult to assemble the rotation transmission device. Further, the spring clutch tends to increase operational torque due to sliding torque of the clutch, or produce noise and cause other problems due to malfunction of the clutch.